Exploring variant-to-variant coercion

[zth]

Allow coercing between variants which have the same exact constructors. We need to ensure that the runtime representation exactly matches as we coerce. The following restrictions are therefore in place:

• Both variants must have the same configuration for @unboxed and @tag
• Each constructor pair must have the same configuration of @as

[zth]

Probably need to check for unboxed too, right?

We already check that.